<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        .box {
            display: flex;
            width: 400px;
            outline: 1px red solid;

        }

        .item1 {
            flex: 0 2 300px;
            background-color: #32d6d6;
        }

        .item2 {
            flex: 0 1 200px;
            background-color: #e2a83e;
        }

        .item3 {
            flex: 0 2 100px;
            background-color: #b85ad0;
        }
    </style>
</head>
<body>
<div class="box">
    <div class="item1">1</div>
    <div class="item2">2</div>
    <div class="item3">3</div>
</div>
</body>
</html>